The present invention relates to motor-pump assemblies in general, and more particularly to improvements in encapsulated submersible motor-pump assemblies wherein the pump (normally a centrifugal pump) is called upon to circulate a liquid medium which is maintained at an elevated pressure as well as at an elevated temperature. Still more particularly, the invention relates to improvements in pressure equalizing systems for the liquid-filled housings or casings of electric motors which are used in such assemblies to drive the pump shaft.
It is already known to cool the liquid which fills the housing of the electric motor in a motor-pump assembly of the type wherein the pump circulates a liquid medium at an elevated temperature and at an elevated pressure and wherein the mechanical seal for the pump shaft is supplied with a sealing liquid. The seal is disposed between the pump housing or casing and the housing or casing for the electric motor. Such types of motor-pump assemblies are often employed under circumstances where the pump aspirates directly from its surrounding without a suction pipe, i.e., they can be said to constitute so-called submersible motor pumps. Submersible motor pumps often employ pressure equalizing devices which ensure that the pressure in the motor housing matches the pressure around the motor housing or in the pump housing. Pressure equalizing devices of conventional design can be divided into two categories the first of which employs a deformable membrane connected to the motor housing. The space at one side of the membrane communicates with the interior of the motor housing, i.e., one side of the membrane is acted upon by liquid which fills the motor housing. The other side of the membrane is contacted by liquid whose pressure matches or approximates that in the area surrounding the motor housing. To this end, the latter is formed with slots or other types of openings which may but need not contain check valves and serve to admit liquid from the area around the motor housing into contact with the other side of the membrane. Reference may be had to U.S. Pat. No. 2,962,612 granted Nov. 29, 1960 to Lung, to German Auslegeschrift No. 11 86 141 and/or to German Utility Model No. 18 45 050. The membrane can be said to constitute an elastic wall which takes care of pressure equalization, i.e., it ensures that the pressure of liquid in the motor housing matches the pressure of liquid around such housing.
A different pressure equalizing device is disclosed, for example, in U.S. Pat. No. 3,241,492 granted Mar. 22, 1966 to Arutunoff. This patent discloses an elastically deformable receptacle which is disposed externally of the motor-pump assembly and is connected with the interior of the motor housing by a suitable conduit. The principle of operation is the same as in assemblies wherein a membrane is disposed in the interior of or on the motor housing, i.e., the external surface of the deformable receptacle is acted upon by liquid which surrounds the motor housing and this ensures that the pressure of liquid in the motor housing (and in the interior of the deformable receptacle) matches the external pressure. The receptacle expands when the pressure in the motor housing rises above the pressure of liquid around the motor-pump assembly whereas, when the external pressure rises, the receptacle contracts and some of its contents are caused to flow into the motor housing.
It is further known to employ a pressure equalizing device which embodies a cylinder and piston unit as a substitute for the aforementioned membrane or deformable receptacle. The cylinder and piston unit is installed in a submersible motor-pump assembly with an electric motor, e.g., in a manner as disclosed in German Pat. No. 10 50 434. All of the aforediscussed conventional assemblies share the characteristic that the temperature of liquid in the motor housing matches or approximates the temperature of liquid in the surrounding area, i.e., the temperature at one side of the membrane is the same as, or it closely approximates that, at the other side of the membrane; the temperature in the deformable receptacle at least approximates that in the area around the receptacle; and the temperature of liquid in the cylinder and piston unit and in the motor housing on the one hand matches or approximates the temperature of liquid medium around the motor housing.
It has been found that the aforediscussed pressure equalizing devices are not entirely satisfactory in all situations which necessitate the use of submersible motor pumps, especially when the pump is required to circulate a liquid which is maintained at an elevated pressure as well as at an elevated temperature. The just outlined situations often arise in the processing of crude oil or in the liquefaction of carbon. As a rule, such motor-pump assemblies employ shaft seals in the form of bellows which seal the motor housing from the pump housing. The seal is protected from the adverse influence of solid particles by a clean sealing liquid which is fed into the pump housing and which also serves to cool the seal, i.e., to remove friction heat. The seal is designed in such a way that it opens in response to establishment of a pressure differential of the sealing liquid and that it closes when the delivery of sealing liquid is interrupted.
The liquid-filled motor is combined or equipped with a device which circulates highly pressurized liquid through a cooler. This ensures that the temperature of the motor is maintained within acceptable limits. As a rule, the means for circulating the liquid which fills the motor housing includes a rotor or impeller on the shaft which drives the impeller or impellers of the pump. When the motor is arrested and the just mentioned rotor comes to a halt, the liquid which fills the motor housing continues to circulate by flowing from the upper part of the motor housing, through the cooler, and into the lower part of the motor housing. When the delivery of sealing liquid is interrupted and the motor is idle, such natural circulation of liquid contents of the motor housing causes a reduction of the temperature of liquid in the motor housing so that the density of liquid in the motor housing increases with the result that the pressure drops below the pressure in the pump housing while the shaft seal is closed.
A similar situation can arise while the motor is running, i.e., the pressure of liquid which fills the motor housing can drop below the liquid pressure in the pump housing. This can take place, for example, in response to manipulation of the system which admits the sealing liquid. Still further, a change in the speed of the motor can entail a change of density of the liquid which fills the motor housing, primarily as a result of a change in temperature of such liquid. Under each of the just outlined circumstances, the pressure differential between the interior of the motor housing and the interior of the pump housing endangers the shaft seal as well as the thrust bearing for the motor shaft. For example, and in the absence of any remedial measures (such as the provision of a pressure equalizing device), the aforediscussed bellows of the shaft seal would be destroyed after a short period of operation during which the difference between the pressures of liquids in the motor housing and the pump housing exceeds a certain value, and the thrust bearing for the motor shaft would be subjected to excessive stresses.
Furthermore, under the aforedescribed severe circumstances, conventional pressure equalizing devices with membranes in the motor housing or with externally mounted elastically deformable receptacles are incapable of adequately protecting the shaft bearing and/or the thrust bearing for the motor shaft. The reasons for failure of such conventional pressure equalizing devices under the aforediscussed circumstances are manifold and include the utilization of motor-pump assemblies in a different environment but particularly the pronounced difference between the temperature of liquid in the pump housing on the one hand and the temperature of liquid in the motor housing on the other hand. Thus, if one would attempt to bring about an equalization of pressures which prevail in the two housings in a manner known from conventional pressure equalizing devices, this would necessarily result in an equalization or near equalization of temperatures. In other words, the temperature of liquid in the motor housing would be raised to the elevated temperature of liquid in the pump housing with the result that the useful life of temperature-sensitive component parts of the motor would be greatly reduced and the motor of the motor-pump assembly would necessitate frequent inspection, repair or replacement with attendant greatly increased maintenance cost and prolonged interruptions of operation.